Mag._,
Sept., 1888) reproducing with remarkable exactness most of the
features above described. In order to explain the vigor and purity of
the color reflected in certain crystals, it is necessary to suppose
that there are a considerable number of twin surfaces disposed at
approximate equal intervals. At each angle of incidence there would be
a particular wave length for which the phases of the several
reflections are in agreement. The selection of light of a particular
wave length would thus take place upon the same principle as in
diffraction spectra, and might reach a high degree of perfection.
In illustration of this explanation an acoustical analogue is
exhibited. The successive twin planes are imitated by parallel and
equidistant disks of muslin (Figs. 1 and 2) stretched upon brass rings
and mounted (with the aid of three lazy-tongs arrangements) so that
there is but one degree of freedom to move, and that of such a
character as to vary the interval between the disks without disturbing
their equidistance and parallelism.
The source of sound is a bird call, giving a pure tone of high pitch
(inaudible), and the percipient is a high-pressure flame issuing from
a burner so oriented that the direct waves are without influence upon
the flame (see _Nature_, xxxviii., 208; Proc. Roy. Inst., January,
1888). But the waves reflected from the muslin arrive in the effective
direction, and if of sufficient intensity induce flaring.
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